What is the third switch terminal for?

@cecil

Maybe t his will help ...

https://wiringsolver.com/wire-a-3-prong-lighted-toggle-switch/

@cecil Do they have a light? 

@cecil I looked at @will's link and I don't get it, I would use a VOM to determine what each lead does. BTW, I used to be an electrician.

Posted by: @zander

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@cecil Do they have a light? 

Judging from the top picture, the ones in the middle and on the right appear to have a translucent rocker which suggests (to me at least) that they are lighted from within.

The one on the left is not as obviously lit internally, but I'm guessing that if it really is a SPST, then the third leg is for an internal light.

@will @cecil The one on the left is labelled as 125VAC and I suspect the others are as well. That being the case, determining the connections with a VOM is more critical. Since they all seem to have legible markings, I would google anything that looks like a part number which will most likely give you the exact connection details.

@zander

I suspected the switches were lighted and now I see "LIGHT" written vertically next to the third terminal of the push button switch.

I used an ohm meter to determine the switched terminals and confirmed that the third terminals do not connect to either of the switched terminals whether the switch is on or off.  That eliminates a  bulb filiment but I guess not an LED.

All the switches are labeled with current capacity at 120VAC, 250VAC and 28VDC.  There are also manufacturer names, patent numbers, and UL listing claims but no part numbers.

The WiringSolver article states the third terminal should go to ground, which may also mean neutral.  That is a possibility I did not think to check so I will do that now.

@cecil The wiring diagram I saw looked bizarre to this old electrician. If you can identify the two pins that when a VOM is connected to them display closed or open when the switch is toggled, then those two terminals get wired into the hot leg of the circuit you are controlling. Then just connect the other terminal to first the one pin and then the other. If neither of those connections works the way you want, reverse the two pins carrying the power and try again.

@cecil Test with a battery, NOT AC.

Posted by: @zander

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Then just connect the other terminal to first the one pin and then the other. If neither of those connections works the way you want, reverse the two pins carrying the power and try again.

I'm still stumped.  I did the tests stated in the quote and none of the switches lit up.  I tried with 12VDC, 24VDC and then with 120VAC through a lightbulb current limiter.

So either there is some other wiring scheme required or the lights in all three switches are defective.  These are new, old stock switches I bought from a liquidator 15 years ago.

I did find some part numbers on the packaging--it took a magnifying glass to make out the faded dot-matrix print.  Alas, Google did not find any documents.

I do not need a lighted switch for my project since a volt meter will light up when the power is switched on.  I just hate to give up on mysteries when I know there is a scientific answer.

@cecil I assume the basic switch works, just the light doesn't work. Maybe that's why they were being liquidated.

Hi @cecil,

  One switch (on the left) I could read had 125Vac. No mention of any other voltage.

  As I live in country with 230 Vac and a different earth/neutral standard of wiring, I have been reluctant to comment. So please take this as a mixture of my local experience and guesswork how it might be in the US. But please bear in mind, this is guesswork on my part.

As suggested, these look like they should have illumination, but in the UK at least, this would typically be with a small neon lamp and series resistor. Logically this would be connected between the switch live out, and neutral/earth (I am not clear what your naming convention is for the latter) .

Unfortunately, as the neon is a gas filled tube, and needs around 90V to strike, you can't find it with a general multimeter. Furthermore, it probably qualifies for the title of one of the most unreliable bits of general purpose mains wiring in my country. The neon tubes themselves can die of old age, particularly if they have been used in a position that means they are on 24 x 7. Furthermore, the resistor and neon have thin wire connections, that often just get tucked into a small space alongside the main switch connection metalwork, in the hope that they will touch and make contact, rather than being securely connected. Hence they work for a while, then stop working or become intermittent.

It is strange you say one switch mentions 3 different voltage/current combinations. Maybe that one doesn't have a light .. or perhaps the body of the switch was designed without the thought a light of some type could be fitted, and the writing only applies to the switch contacts.

To be honest, it is difficult to know what to recommend .. especially if you are not used to dealing with mains voltages. These types of switches are only ever designed to be assembled once, so attempts at dissassembly usually weakens or destroys them. And if they do have a neon type lamp, you will need to apply 115V, to the correct terminals, to see if they work, which given the uncertainity, is obviously hazardous and I DO NOT RECOMMEND IT.

Sorry, it is not a positive message, but I would never forgive myself if someone was unnecessarily hurt by my advice. I would consider them best consigned to the appropriate waste recycling bin.

Take care my friends. Best wishes. Dave

Yes the switches work.  The lights do not and that is not a big deal.  So I do not need to make a mystery out of it.

@DaveE,  The naming convention in the US is hot, neutral and ground, or, alternatively, live, grounded and grounding.  When I was a boy there were no ground, or grounding wires run to household receptacles.  I'm not sure when that changed.

Thanks for your help.